The article, which appeared as an online first issue on 29 July in The Lancet is written by Dr Denise J Jamieson, of the Division of Reproductive Health, at the US Centers for Disease Control and Prevention (CDC), in Atlanta, Georgia, and colleagues.
Last month the World Health Organization (WHO) declared that the novel H1N1 swine flu virus that is sweeping the world is a global pandemic. Cases and deaths started being reported from Mexico, followed by the USA, and while experts were expecting the virus to wane in the northern hemisphere during the summer months it has not done so and continues to infect people in the US, as other countries, with more deaths occurring.
For the study, Jamieson and colleagues focus on cases of infection among pregnant women identified during the first month of the outbreak and deaths among pregnant women that were linked with H1N1 swine flu during the first 2 months.
As part of enhanced surveillance, the CDC began systematically to collect data on cases of H1N1 infection in pregnant women soon after initial cases emerged, and the authors used this data, together with population estimates from the 2007 census to calculate rates of illness and admission to hospital.
They defined a confirmed case as:
"An acute respiratory illness with laboratory-confirmed pandemic H1N1 virus infection by real-time reverse-transcriptase PCR or viral culture."
And a probable case as:
"An acute febrile respiratory illness who was positive for influenza A, but negative for H1 and H3."
The results showed that:
- Between 15 April and 18 May 2009 (the first month of the outbreak), the CDC received reports of 34 confirmed or probable cases of pandemic H1N1 in pregnant women from 13 states.
- 3 of the women were in their first trimester, 19 were in their second, 9 were in their third, while the trimester stage of the other 3 was unknown.
- 5 women were under 18 years of age, 17 women were between 18 and 29 years, 11 were aged between 30 and 39 and one was over 40.
- 9 of the women were non-Hispanic white, 2 were non-Hispanic black, 15 were Hispanic, 1 was Asian, 1 was American Indian/Alaskan Native, and 6 were of unknown race.
- 4 of the women had been in Mexico in the previous 7 days, 27 had not, while there was no information about travel on the other 3.
- 11 of the cases (32 per cent) were admitted to hospital.
- During the first month of the outbreak, the estimated rate of hospital admission for pregnant women infected with pandemic H1N1 virus was higher than it was in the general population.
- For pregnant women the hospitalization rate in the first month was estimated to be 0·32 per 100,000 and for the general population it was 0·076 per 100,000.
- Between 15 April and 16 June 2009 (the first two months of the outbreak), the CDC received reports of 6 deaths among pregnant women, all of whom had developed "pneumonia and subsequent acute respiratory distress syndrome requiring mechanical ventilation".
Nevertheless, the authors interpreted the findings as suggesting that:
"Pregnant women might be at increased risk for complications from pandemic H1N1 virus infection."
The authors suggest these findings support the present recommendation to:
"Promptly treat pregnant women with H1N1 influenza virus infection with anti-influenza drugs."
Although the decision to admit a pregnant woman is not a simple one, wrote the authors, and might include considerations beyond the severity of the illness, they said it was still a worry that over 10 per cent of the influenza-related deaths in the US have been in pregnant women.
This trend appears to be shared with the 1918 and 1957 pandemics, when mortality among pregnant women was also higher than in the general population.
"During the pandemic of 1957, 50 percent of deaths due to Asian influenza in Minnesota among women of reproductive age occurred in pregnant women, " wrote the authors.
They also noted that while the CDC recommends that pregnant patients be given antivirals (they mentioned two in particular) as soon as possible after the onset of flu, because the benefit is expected to be greatest if the drugs get to work within 48 hours of onset, many of the pregnant women included in the study:
"Were not treated with either of these drugs at the time of their presentation with influenza-like illness."
"Furthermore, none of those who died were treated within 48 h of illness onset," they added.
The authors added that in view of the anticipated effects of novel H1N1 on pregnant women, the benefits of giving them antivirals are likely to to outweigh potential risks to their unborn baby.
However, since most of the women included in this study are still pregnant, as most of the other women infected elsewhere in the world, it is not possible to say how the novel H1N1 virus and treatments for it might have affected the fetus.
The authors also noted that while vaccination, once available, "will be an essential component of the public health response to this influenza, and US guidelines place pregnant women in a high-priority group for receipt of pandemic influenza vaccine," they expressed concern that few women actually come forward to be vaccinated for seasonal flu when they are pregnant.
Among their concluding remarks they wrote:
"Findings from this study will be crucial to inform public health planning for pregnant women, both for this virus and for other novel pathogens."
"Crucially, health-care providers have to realise that pregnant women are at increased risk for severe disease and complications from pandemic H1N1 influenza virus infection, and should start treatment with anti-influenza drugs promptly," they added.
"H1N1 2009 influenza virus infection during pregnancy in the USA."
Denise J Jamieson, Margaret A Honein, Sonja A Rasmussen, Jennifer L Williams, David L Swerdlow, Matthew S Biggerstaff, Stephen Lindstrom, Janice K Louie, Cara M Christ, Susan R Bohm, Vincent P Fonseca, Kathleen A Ritger, Daniel J Kuhles, Paula Eggers, Hollianne Bruce, Heidi A Davidson, Emily Lutterloh, Meghan L Harris, Colleen Burke, Noelle Cocoros, Lyn Finelli, Kitty F MacFarlane, Bo Shu, Sonja J Olsen, and the Novel Influenza A (H1N1) Pregnancy Working Group.
The Lancet, Published Online July 29, 2009.
Additional sources: The Lancet, CDC.